1. Field of the Invention
The present invention relates to a trouble diagnostics apparatus for a fuel treatment system More specifically, the present invention relates to a trouble diagnostics apparatus for a fuel treatment system preferably usable for diagnosing a leak in a system including a fuel tank.
2. Background Art
An example of a trouble diagnostics apparatus for diagnosing a leak in a system including a fuel tank is described in Japanese Patent Laid-open No. 2002-4959. This trouble diagnostics apparatus includes a vacuum pump for introducing a negative pressure to the fuel tank by drawing through suction a gas contained in the fuel tank. A tank's internal pressure is quickly vacuumized when the vacuum pump is operated, if there is no leak in the fuel tank. If there is a leak in the fuel tank, on the other hand, the particular leak prevents the tank's internal pressure from being vacuumized. According to this conventional trouble diagnostics apparatus, therefore, it is possible to determine whether or not there is a leak trouble if the following is done. Specifically, it is determined whether or not the tank's internal pressure is properly vacuumized after the lapse of a predetermined period of time, through which a steady-state pressure is reached, after the vacuum pump has been started.
In the conventional trouble diagnostics apparatus, however, a steady-state pressure reaching time through which the tank's internal pressure reaches the steady-state pressure is not constant. Accordingly, if an approach is taken in which a decision is made whether or not there is a leak trouble after the lapse of the predetermined steady-state pressure reaching time using this conventional device, the following problems could result. Specifically, it may be determined whether or not there is a trouble at a stage before the tank's internal pressure is yet to be decreased sufficiently to reach the convergence value. Or, the vacuum pump is kept running unnecessarily after the tank's internal pressure has already reached the convergence value.